1. Field of the Invention
The present invention relates to a magnetic circuit for a speaker for use in various audio equipments, and more particularly to a magnetic circuit using a sintered magnet, a method of manufacturing the circuit, and a speaker incorporating the magnetic circuit.
2. Description of Related Art
Since speakers used in televisions and radios are usually demanded to be small in size and low in profile, an inner magnet type magnetic circuit is adopted, which comprises a pot yoke, a sintered magnet, and a pole piece. The magnetic circuit using a sintered magnet involves a disadvantage that a pot yoke is required. In order to reduce the profile of the magnetic circuit, the pot yoke, sintered magnet and pole piece must all be lower-profiled. Since the performance of the magnetic circuit is dependent on the thickness of the pot yoke and the pole piece, if the pot yoke and the pole piece have their profiles reduced while the sintered magnet has its thickness kept unchanged, the magnetic circuit is saturated thereby failing to achieve a satisfactory performance, and if a plurality of voice coils are used, the voice coils require respective magnetic circuits.
In this connection, Japanese Patent Application Laid-open No. Sho 63-99700 discloses a magnetic circuit, in which a sintered magnet shaped annular and having anisotropy in the radial direction is integrated with a pot yoke, whereby a speaker can be lower-profiled and the magnet can be more efficient.
The magnetic circuit disclosed in Japanese Patent Application Laid-open No. Sho 63-99700 comprises: a yoke, which has a center pole, a flange extending outward from the bottom of the center pole, a first stepped portion formed on the flange at a prescribed distance from the center pole and protruding in the direction opposite to the center pole, and a second stepped portion formed at the outer circumference of the flange and protruding in the same direction as the center pole; and a sintered magnet, which is shaped annular, has an anisotropy in the radial direction, is disposed on the flange of the yoke, has its inner circumference sintered to be integrated with the outer circumference of the center pole, has an outer circumference with a diameter larger than the diameter of the first stepped portion, and which, in association with the second stepped portion, forms an air gap with a predetermined dimension.
The magnetic circuit above described is effective to some extent in that a pole piece disposed over the sintered magnet can be eliminated thereby achieving a lower-profile speaker, but is not satisfactorily effective because it still requires a yoke which takes up the largest space. Also, the magnetic circuit has its yoke structured extremely peculiar as above described, and the yoke has its flange sintered to be integrated with a sintered magnet, thus the fabrication cost of the magnetic circuit is not reduced.